The United Nations' human rights chief has again called on Syrian authorities to allow international observers to enter the country, warning that Syria is close to moving into full civil war.

Speaking to reporters December 9, UN High Commissioner For Human Rights Navi Pillay backed estimates saying that nearly 1,000 members of the Syrian security forces have been killed in the clashes that have taken place since March, when protests began against President Bashar al-Assad's regime.

The UN says more than 4,000 people on all sides have died in the nine-month conflict.

"I acknowledge that almost 1,000 of President Assad's security forces have also been killed in this conflict,” Pillay said at UN headquarters in New York. “This is why I am alerting the world that as you have more and more defectors from the security forces, this may well develop into a fully fledged civil war."

Syrian anti-regime activists on December 9 reported that Syrian security forces had fired on fresh anti-government demonstrations across the country, killing at least 24 people, including a number of children.

The toll could not be independently confirmed, as the Syrian regime has banned most foreign journalists from the country and prevents local reporters from moving around freely.